1. Field of the Invention
This invention relates to a novel image forming method employing a photosensitive member comprising an organic photoconductive member having a specific surface hardness and a developer utilizing magnetic powder having a specific BET specific surface area.
2. Description of the Prior Art
As the photoconductive material to be used in electrophotographic photosensitive, inorganic materials such as selenium, cadmium sulfide and zinc oxide have been known. These photosensitive members have a number of advantages such as capability of charging to an appropriate potential in the dark, small dissipation of charges in the dark or rapid dissipation of charges by light irradiation. On the other hand, they also possess various drawbacks. For example, in selenium photosensitive members, crystallization will readily occur due to factors such as temperature, humidity, dust, pressure, etc. Particularly, at an atmospheric temperature in excess of 40.degree. C., crystallization will proceed markedly to cause deleterious lowering in charging characteristic or occurrence of white speckles. Also, in a selenium photosensitive member or cadmium sulfide photosensitive member, there is involved the drawback that no stable sensitivity and durability can be obtained in prolonged usage under humid conditions.
On the other hand, a zinc oxide photosensitive member requires a sensitizing effect with a sensitizing dye, typically Rose Bengal. Since such a sensitizing dye will suffer deterioration by corona charging or color fading by exposed light, there is a drawback in that no stable image can be given over a long term.
Various organic photoconductive polymers, typically polyvinylcarbazole, have also been proposed, but these polymers, while they are superior to those inorganic photoconductive materials as mentioned above with respect to film forming property, light weight, high productivity, etc., could hitherto hardly been practically applied, because they are inferior to inorganic photoconductive members with respect to sensitivity, durability characteristics and stability to environmental changes. Moreover, no appropriate sensitizer capable of sensitizing sufficiently the organic photosensitive member has yet been found.
For the reasons as mentioned above, a number of developments have also been made in recent years in low molecular weight organic photoconductive materials in place of polymeric organic photoconductive materials. The advantage of low molecular weight organic photoconductive materials resides in broadened scope of suitable compounds, which enables choice of compounds having high sensitivity and charge retentivity, whereby the drawbacks of the photosensitive member caused by use of the organic photoconductive polymer of the prior art can be removed.
The photosensitive member comprising an organic photoconductive material (hereinafter abbreviated as OPC) has the drawback that it has a low surface hardness and hence is susceptible to scratches. For this reason, the surface of the photosensitive member cannot be cleaned strongly and therefore the low resistance materials formed on the photosensitive member surface by corona charging, etc., paper powder attached on the surface or other low resistance materials formed by other causes are difficult to remove. In particular, under high temperature and high humidity conditions, the aforesaid materials will absorb moisture until resistance is extremely lowered to disadvantageously disturb latent images.
As the result of various investigations on the method to overcome such drawbacks of OPC photosensitive members, it has now been found that it is effective to bring a magnetic toner or magnetic particles containing specific magnetic powder into contact with an OPC photosensitive member.
A novel developing method has previously been proposed in Japanese Laid-open Patent Publication No. 43036/1979. This method comprises thinly applying a magnetic toner onto a sleeve, subjecting this to triboelectric charging, then effecting development by bringing this toner under the action of a magnetic field to confront the electrostatic image without contact. According to this method, excellent images can be obtained because of such reasons that sufficient triboelectric charging is rendered possible by thinly applying the magnetic toner on the sleeve thereby increasing the chances of contact between the sleeve and the toner, that the toner is supported by magnetic force, and the magnet and the toner are moved relative to each other whereby mutual agglomeration between toner particles is disintegrated and also sufficient friction with the sleeve is attained, and that ground fog is prevented by supporting the toner by magnetic force and performing development while bringing the toner closely face to face to the electrostatic image without contact. However, according to this method, in spite of the use of a magnetic toner, the above-mentioned disturbance of the latent image could hardly be cancelled, when it is applied for an OPC photosensitive member.